Proteins which contain dinuclear metal clusters are essential for many basic processes of life, including DNA synthesis, oxidative phosphorylation, methane oxidation, fatty acid degradation, peroxide detoxification, and oxygen and electron transport. In addition, the nucleic enzyme ribozyme, which catalyses a self cleavage reaction, is believed to contain a dinuclear metal cluster at the active site. Spectroscopic studies of dinuclear metal cluster contribute to our understanding of the function of these enzymes and proteins at an atomic level. We have developed new spectroscopic instrumentation and quantitative methodologies specifically suited to probe these metal cluster. EPR spectroscopy has long been a valuable tool for the study of the transition-metal ions that are found in the active centers of many proteins. Many metals of biological importance such as vanadium, chromium, manganese, iron, cobalt,, nickel, copper, and molybdenum, have unpaired electrons that give rise to a new electronic spin and a resultant magnetic moment. EPR spectroscopy is capable of measuring this moment and therefore provides a microscopic probe for the active sites of these proteins. For many years, EPR spectroscopy has been limited to studies of metalloproteins have an odd number of unpaired electrons in at the metal. However, our work over the past years has significantly broadened the range of applications for EPR spectroscopy to include quantitative analysis of metalloproteins with an even number of unpaired electrons. In particular, the oxidation states of dinuclear metal clusters that are of main biological importance, are states that predominantly have an even number of electrons.